US Secretary of State John Kerry Wednesday denounced the beheading of journalist Steven Sotloff as an act of "medieval savagery" and said the jihadists who killed him were not the true face of Islam.

"The real face of Islam is not what we saw yesterday, when the world bore witness again to the unfathomable brutality of ISIL terrorist murders," Kerry said at a ceremony to honor the new special US envoy to Muslim communities.

The top US diplomat called the 31-year-old Sotloff "a driven and courageous journalist," saying he was killed by a "coward hiding behind a mask" from the Islamic State militants also known by the acronym ISIL.

After having worked to try to bring Sotloff home safely, Kerry said his murder so soon after that of fellow reporter James Foley last month was "a punch to the gut."

The United States would hunt down Sotloff's killers, Kerry vowed.

"Those who have murdered James Foley and Steven Sotloff in Syria need to know that the United States will hold them accountable... no matter how long it takes," the veteran diplomat said to applause at the State Department ceremony for new envoy Shaarik Zafar.

"The face of Islam is not the butchers that killed Steven Sotloff. That's ISIL. The face of Islam is not the nihilists who know only how to destroy not to build. It's not masked cowards, whose actions are an ugly insult to the peaceful religion that they violate every single day with their barbarity," Kerry insisted.

"The real face of Islam is a peaceful religion, based on the dignity of all human beings. It's one where Muslim communities are leading the fight against poverty, it's one where Muslim communities are providing basic healthcare and emergency assistance on the frontlines of some of our most devastating humanitarian crises."

Kerry also paid tribute to Sotloff, saying his "reporting was as empathetic as his killers are evil" and comparing him to legendary 20th century war correspondent Martha Gellhorn.

"He focused on the stories of average people trapped in war, and documented their day-in and day-out struggle for dignity," Kerry said.

"Like Martha Gellhorn, he chronicled humanity in the face of inhumanity, and he told the story of enormous generational events as if they were happening to someone you knew from your own life."

US Secretary of State John Kerry Wednesday denounced the beheading of journalist Steven Sotloff as an act of medieval savagery and said the jihadists who killed him were not the true face of Islam. The real face of Islam is not what we saw yesterday, when the world bore...